UPDATES ON MONITORING DEPTH OF ANESTHESIA

Abstract

Unintended intraoperative awareness is a dreaded complication of anesthetic practice that is associated with a high rate of posttraumatic stress disorder (PTSD). Most anesthetics cause antegrade amnesia (loss of memory for a period after administration of the drug) but not retrograde amnesia (loss of memory for events preceding administration of the drug).Intravenous anesthetics, including propofol and etomidate, cause antegrade amnesia and can also interfere with memory consolidation, which refers to the stabilization of memories after the initial acquisition. Intraoperative awareness is defined by both consciousness and explicit memory of surgical events. It occurs in 1 or 2 of every 1, 000 surgical cases, but incidence varies with the patient population, methodology used to study awareness, and time frame of the study. Risk factors include compromise of cardiovascular function as well as acquired or inherited resistance to the sedative or amnesic effects of anesthesia. Electroencephalographic techniques to detect and prevent awareness. The best and main monitor in the operating room is always the anesthesiologist. An anesthesiologist could monitor whole of the patient’s condition and follow the course of the surgery, anticipating problems and correcting them as and when they occur. The vigilant anesthesiologist continuously obtains subjective and objective information from the anesthetized subject. Subjective monitoring depend on the anesthesiologist’s senses (visual, tactile, auditory, ‘sixth sense’) and the experience. By contrast, even the most sophisticated electronic monitors are inherently limited. They can monitor only one aspect of the patient’s condition. They require electrical power, need regular maintenance, occasionally develop faults and are prone to error. Their advantage is that, they do not succumb to stress, boredom, fatigue, and distraction. Most of thedevices designed to monitor brain electrical activity for the purpose of assessing anesthetic effect record EEG activity from electrodes placed on the forehead. Systems can be subdivided into those that process spontaneous EEG and EMG activity and those that acquire evoked responses to auditory stimuli i.e. auditory evoked potential (AEPs). After amplification and conversion of the analog EEG signal to the digital domain, various signalprocessingalgorithms are applied to the frequency, amplitude, latency, and/or phase relationship data derived from the raw EEG or AEP to generate a single number, often referred to as an “index, ” typically scaled between 0 and 100. This index represents the progression of clinical states of consciousness (“awake, ” “sedated, ” “light anesthesia, ” “deep anesthesia”), with a value of 100 being associated with the awake state and values of 0 occurring with an isoelectric EEG (or absent middle latency AEP). We have presented modelled cost-effectiveness analyses for BIS, E-Entropy and Narcotrend compared with standard clinical monitoring, for two modes of anesthetic administration. There is substantial uncertainty associated with the analysis, given the weakness of the evidence base for the majority of outcomes included in the model. No robust evidence was identified on the effectiveness of E-Entropy or Narcotrend in avoiding intraoperative awareness or POCD and, in the absence of such evidence; we have assumed that the effect estimates derived for BIS, can be applied. However, even in the case of BIS, the evidence base is currently severely lacking. There is also limited evidence on the baseline incidence of anesthetic complications included in the model. There is more evidence on the benefit in terms of reduced anesthetic drug consumption, although for some technologies the evidence is inconclusive. Overall, the economic evaluation indicates that, for general surgical patients, some of the additional costs of depth of anesthesia monitoring may be offset by reduction in consumption of anesthetic drugs.